Low-Altitude Wind Shear and Its Hazard to Aviation (1983) / Chapter Skim
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2. Low-Altitude Wind Shear
2. Low-Altitude Wind Shear
Pages 19-50
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From page 19... ...
To eliminate -- or at least appreciably reduce -- the hazards posed by low-altitude wind variability, it is necessary to understand the sources of wind changes and the risks they represent. It is also necessary to know how to detect, measure, and predict them and how to communicate useful information on wind variability to air traffic controllers and pilots in a timely fashion.
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From page 20... ...
The following paragraphs describe the types of wind-shear situations and the risks each poses to aviat ion. Convective Outflows Thunderstorms and other convective clouds are critically important sources of low-altitude wind variabil ity.
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~-to-~[o ~ of ~ :,l: J~^~ a:' ~~::-1~ ~ ,,% a- I ~ - ~ lo Ad \ ~ q'~ 1 -- ~- \, ~ .
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From page 22... ...
PAM automatically measures wind speed and direction, dry- and wet-bulb temperatures, and pressure and rainfall at 27 surface stations (wind measurement is at 12 feet above the ground)
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From page 23... ...
~ Source: Fuj its and Wakimoto, 1983 ~ . Aircraft passing through the center of a microburst will experience a change in wind velocity that can be specified as the vector difference of the headwind and the tailwind along the flight path.
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From page 24... ...
Nevertheless, the Doppler radars detected and observed 75 microbursts on 33 of the 86 operational days (McCarthy et al., 1983~. The Doppler radar data allowed for an examination of the time history of JAWS microbursts observed near Denver.
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~ ~ ~ ~ ? ~ ~ ,, 1 l4 `_~ i _ _ , , 7 8 9 10 DISTANCE EAST OF CP-2 (miles} (CP-2, S-Band Doppler Radar Site)
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From page 26... ...
(Source: Wilson and Roberts, 1983~. Gust Fronts A gust front is the leading edge of a mass of cool air that has recently descended from a thunderstorm or convective cloud.
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From page 27... ...
Also, radar echoes often indicate the presence of deep convective clouds and the possibility of a gust front. Gravity and/or Solitary Waves As a gust front moves away from its parent s form, the temperature contrast across it is gradually reduced.
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From page 28... ...
Wind shear associated with sea breezes can prove to be a hazard at airports that are located along coastlines, such as Logan International Airport in Boston and the John F Kennedy International Airport in New York City.
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From page 29... ...
The following pilot report is typical of a flight through a frontal zone with high wind variability. It was recorded at the Madison, Wisconsin, airport.
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From page 30... ...
The NWS provides forecasts for low-altitude wind variability for warm and cold fronts, low-altitude jet and nocturnal inversions, cold surface inversions, friction-surface slowing, inversions, and sea-breeze fronts. Weather forecasters cannot now specify the altitude of the base of the layer of strong wind shear beyond 4 hours.
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From page 31... ...
It is not possible, however, to precisely forecast the steady-state wind velocity, the gust factor, or the turbulence intensity of such winds. These have been observed during every month of the year in Alaska and in the western mountainous regions of the United States and Canada.
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From page 32... ...
j j it 1(~( <~ ~J~J~ V~ ] '~' ~ 271 267 1200 27 18002724 240024 24 21 0600 1200 June 26 CENTRAL STANDARD TIME June 27, 1965 FIGURE 8 Low-Altitude Jet Stream Near Norman, Oklahoma, Observed by Means of Doppler Radar.
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From page 33... ...
Because of the very s bong wind shears that characteriz e them, tornadoes should always be avoided. Most of the time they can be seen by pilots flying below the bases of the parent thunderstorms.
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From page 34... ...
This may not be a big problem for gust fronts or sea breezes, but it limits the detection of wind shear that may not be present at the surface. o There are temporal and spatial resolution limitations that may present serious problems for detecting the smallest-scale events.
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From page 35... ...
Because the sensors were located over a wide area during the JAWS Project, PAM was able to measure the wind speed and direction on opposite sides of microbursts occurring within its field and thus provide evidence of the existence of the microburst.
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It must be clearly understood that the microburst daily frequency statistics shown on the left side of Figure lO do not include other wind-shear events, such as gust fronts. Consequently, the data presented here illustrate only the inadequacy of the LLWSAS as a microburst detector.
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From page 37... ...
During this brief test, the downdraft air in several small microbursts impacted the ground between the sensors, and the resulting gust fronts passed over the sensor array without triggering the pressure sensors. The Atlanta investigations did not adequately test the effectiveness of a combined wind-pressure sensor system for the detection of low-altitude wind shear.
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From page 38... ...
For traffic controllers to use such information, the data must be displayed to them in an appropriate and timely fashion. When mature, intense thunderstorms with heavy precipitation, which causes strong radar echoes, occur over or near an airport, hazardous 10w-altitude wind shear and turbulence are likely~to be present.
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From page 39... ...
, as well as field experiments, have demonstrated that an appropriately designed, pulsed Doppler radar can detect tornadoes, downbursts, microbursts, gust fronts, solitary waves, and sea-breeze fronts. It can also identify the boundaries between distinct air masses (e.g., Wilson et al., 1980~.
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From page 40... ...
Referred to as the next generation radar, or NEXRAD, it will address the common need among the principal government users of meteorological data for information on the current location, severity, and movement of such weather phenomena as tornadoes, severe thunderstorms, heavy precipitation, tropical cyclones, hail, high wind shears, and severe turbulence. Each NEXRAD installation will provide weather information for ranges exceeding 200 miles and heights of 60,000 feet at a rate of once every 3.5 minutes.
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From page 41... ...
An analysis of wind-shear-related accidents shows that the turbulent airflows associated with convective clouds and thunderstorms represent major hazards. The design of a Doppler radar system must take into account the nature of microbursts, specifically their small dimensions, short durations , and the fact that the s tronges t winds may be at altitudes of a few hundred feet.
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From page 42... ...
A microwave Doppler radar operating at a 10-centimeter wavelength proposed for NEXRAD can ef fictively measure wind shear around an airport. What is not known is whether a 10-centimeter wavelength is best for Doppler radars used at airport terminals.
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From page 43... ...
Doppler radar might be used instead of a pulsed Doppler radar to detect low-altitude wind shear. FM-COO Doppler radar has many of the characteristics of the pulsed Doppler radars described in the preceding section.
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From page 44... ...
Such sounders are already in use at the Hong Kong and Calgary, Canada, airports . Airborne Remote Sensing of Wind Shear Microwave Doppl er Radar An airborne Doppler radar would be an effective tool for detecting and warning of hazardous wind shears if it had capabilities equivalent to those available in ground-based systems.
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From page 45... ...
Doppler lidar that is f town on a smal 1 je t aircraft (HS 125 ~ and was tested during the JAWS Pro ject. The system is focused to measure the wind at a range of approximately 1000 feet ahead of the aircraft.
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From page 46... ...
For example, a Doppler radar is a sensor, an essential ingredient for detecting wind shears. Computation is also essential and in a Doppler radar system would provide the signal and data processor to compute the fields of velocity and various other derived information.
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From page 47... ...
Air traffic controllers and pilots are very busy and cannot be burdened with excess ive data. Neither can they afford to work with data that are insuff icient for them to take effective corrective action in real time.
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From page 48... ...
can be accurately predicted hours in advance, but this is not the case with other types of shear. The hazardous shears caused by the downdrafts associated with convective clouds cannot yet be predicted successfully by either numerical or operational techniques.
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From page 49... ...
Thunderstorms are assumed to have the potential for Great ing local titude wind shear, but only on some days do convective clouds cause hazardous wind shears . Recent research at the NOAA's ERLs indicates some skill in forecasting low-altitude wind shear caused by convective clouds.
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From page 50... ...
These ins Lances are While weather forecasts and in-flight advisories admonish that forecas ts of thunderstorms imply the existence of low-altitude wind shears, pilot reports (PIREPs ~ are the most effective warnings of the actual existence of shear. In fact, until more definitive airborne and remote-sensing systems are instituted, PIREPs may be the single most important safety item in identifying most hazards to aircraft operations.
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